luhn 1.0.0.2: only test isValid, use (most) x-common cases

exercism/problem-specifications#474
exercism/problem-specifications#496
exercism/problem-specifications#522
exercism/problem-specifications#547

In consideration of exercism/problem-specifications#523, we
will not be testing the malformed inputs. They have been commented out.

Closes #469

exercises/luhn/examples/success-standard/src/Luhn.hs

@@ -1,32 +1,17 @@
-module Luhn (checkDigit, addends, checksum, isValid, create) where
+module Luhn (isValid) where
 
-revDigits :: Integral a => a -> [a]
-revDigits n = rem10 : digits
-  where (quot10, rem10) = n `quotRem` 10
-        digits | quot10 == 0 = []
-               | otherwise   = revDigits quot10
+import Data.Char (digitToInt)
 
 luhnDouble :: Integral a => a -> a
 luhnDouble n | n < 5     = n * 2
              | otherwise = n * 2 - 9
 
-luhnDigits :: Integral a => a -> [a]
-luhnDigits = zipWith ($) (cycle [id, luhnDouble]) . revDigits
+luhnDigits :: Integral a => [a] -> [a]
+luhnDigits = zipWith ($) (cycle [id, luhnDouble]) . reverse
 
-checkDigit :: Integral a => a -> a
-checkDigit = head . revDigits
-
-addends :: Integral a => a -> [a]
-addends = reverse . luhnDigits
-
-checksum :: Integral a => a -> a
+checksum :: Integral a => [a] -> a
 checksum = (`rem` 10) . sum . luhnDigits
 
-isValid :: Integral a => a -> Bool
-isValid = (0 ==) . checksum
-
-create :: Integral a => a -> a
-create n | chk == 0  = n10
-         | otherwise = n10 + (10 - chk)
-  where n10 = n * 10
-        chk = checksum n10 `rem` 10
+isValid :: String -> Bool
+isValid s = length digits > 1 && checksum digits == 0
+  where digits = map digitToInt $ filter (/= ' ') s

exercises/luhn/package.yaml

@@ -1,5 +1,5 @@
 name: luhn
-version: 0.9.0.1 # 2016-08-04
+version: 1.0.0.2
 
 dependencies:
   - base

exercises/luhn/src/Luhn.hs

@@ -1,16 +1,4 @@
-module Luhn (addends, checkDigit, checksum, create, isValid) where
+module Luhn (isValid) where
 
-addends :: Integer -> [Integer]
-addends n = error "You need to implement this function."
-
-checkDigit :: Integer -> Integer
-checkDigit n = error "You need to implement this function."
-
-checksum :: Integer -> Integer
-checksum n = error "You need to implement this function."
-
-create :: Integer -> Integer
-create n = error "You need to implement this function."
-
-isValid :: Integer -> Bool
+isValid :: String -> Bool
 isValid n = error "You need to implement this function."

exercises/luhn/test/Tests.hs

@@ -1,79 +1,77 @@
-{-# OPTIONS_GHC -fno-warn-type-defaults #-}
+{-# LANGUAGE RecordWildCards #-}
 
+import Data.Foldable     (for_)
 import Test.Hspec        (Spec, describe, it, shouldBe)
 import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
 
-import Luhn (addends, checkDigit, checksum, create, isValid)
+import Luhn (isValid)
 
 main :: IO ()
 main = hspecWith defaultConfig {configFastFail = True} specs
 
 specs :: Spec
-specs = describe "luhn" $ do
-    describe "standard tests" $ do
-
-      it "check digit" $
-        checkDigit 34567 `shouldBe` 7
-
-      it "check digit with input ending in zero" $
-        checkDigit 91370 `shouldBe` 0
-
-      it "check addends" $
-        addends 12121 `shouldBe` [1, 4, 1, 4, 1]
-
-      it "check too large addends" $
-        addends 8631 `shouldBe` [7, 6, 6, 1]
-
-      -- The reference test cases expect the checksum function to return
-      -- the simple sum of the transformed digits, not their `mod 10` sum.
-      -- In this track, we insist on the `mod 10`. :)
-
-      it "checksum" $
-        checksum 4913 `shouldBe` 2      -- The reference test expects 22.
-
-      it "checksum of larger number" $
-        checksum 201773 `shouldBe` 1    -- The reference test expects 21.
-
-      it "check invalid number" $
-        isValid 738 `shouldBe` False
-
-      it "check valid number" $
-        isValid 8739567 `shouldBe` True
-
-      it "create valid number" $
-        create 123 `shouldBe` 1230
-
-      it "create larger valid number" $
-        create 873956 `shouldBe` 8739567
-
-      it "create even larger valid number" $
-        create 837263756 `shouldBe` 8372637564
-
-    describe "track-specific tests" $ do
-
-      -- This track has some tests that were not included in the
-      -- reference test cases from `exercism/x-common/leap.json`.
-
-      it "checksum 1111" $
-        checksum 1111 `shouldBe` 6
-
-      it "checksum 8763" $
-        checksum 8763 `shouldBe` 0
-
-      it "checksum 8739567" $
-        checksum 8739567 `shouldBe` 0
-
-      it "checksum 2323200577663554" $
-        checksum 2323200577663554 `shouldBe` 0
-
-      it "isValid 1111" $
-        isValid 1111 `shouldBe` False
-
-      it "isValid 8763" $
-        isValid 8763 `shouldBe` True
-
-      it "isValid 2323200577663554" $
-        isValid 2323200577663554 `shouldBe` True
-
-      it "create 232320057766355" $
-        create 232320057766355 `shouldBe` 2323200577663554
+specs = describe "valid" $ for_ cases test
+  where
+    test Case{..} = it description $ isValid input `shouldBe` expected
+
+data Case = Case { description :: String
+                 , input       :: String
+                 , expected    :: Bool
+                 }
+
+cases :: [Case]
+cases = [ Case { description = "single digit strings can not be valid"
+               , input       = "1"
+               , expected    = False
+               }
+        , Case { description = "A single zero is invalid"
+               , input       = "0"
+               , expected    = False
+               }
+        , Case { description = "a simple valid SIN that remains valid if reversed"
+               , input       = "059"
+               , expected    = True
+               }
+        , Case { description = "a simple valid SIN that becomes invalid if reversed"
+               , input       = "59"
+               , expected    = True
+               }
+        , Case { description = "a valid Canadian SIN"
+               , input       = "055 444 285"
+               , expected    = True
+               }
+        , Case { description = "invalid Canadian SIN"
+               , input       = "055 444 286"
+               , expected    = False
+               }
+        , Case { description = "invalid credit card"
+               , input       = "8273 1232 7352 0569"
+               , expected    = False
+               }
+-- This track is not testing these cases, since we would rather focus on the algorithm,
+-- and because it seems strange to be unable to distinguish between well-formed invalid input and malformed input.
+--        , Case { description = "valid strings with a non-digit included become invalid"
+--               , input       = "055a 444 285"
+--               , expected    = False
+--               }
+--        , Case { description = "valid strings with punctuation included become invalid"
+--               , input       = "055-444-285"
+--               , expected    = False
+--               }
+--        , Case { description = "valid strings with symbols included become invalid"
+--               , input       = "055£ 444$ 285"
+--               , expected    = False
+--               }
+        , Case { description = "single zero with space is invalid"
+               , input       = " 0"
+               , expected    = False
+               }
+        , Case { description = "more than a single zero is valid"
+               , input       = "0000 0"
+               , expected    = True
+               }
+        , Case { description = "input digit 9 is correctly converted to output digit 9"
+               , input       = "091"
+               , expected    = True
+               }
+        ]